This invention relates generally to computed tomography (CT) imaging and more particularly, to correction of z-axis x-ray beam movement in an imaging system.
In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
At least one known CT system uses a real-time z-axis beam sensing detector to measure the position of the x-ray beam for each view. From the measured position, an error signal representative of the difference between the measured and desired position is determined. Using the error signal, the position of a collimator may be adjusted to reduce the z-axis error between the measured and desired positions. However, the measured position signal at each view contains noise which may have a standard deviation approaching the z-axis error. Although the noise may be filtered, the filtering causes a phase lag and a position error in following the dynamic movement during the scan. As a result, a compromise must be made between loop response time and beam position measurement noise resulting in significant tracking errors.
Accordingly, it would be desirable to provide a system to facilitate correction of z-axis x-ray beam movement. It would also be desirable to provide such a system which improves image quality without increasing patient dosage.